The technical field of this invention is vehicle brake systems.
Brake by wire systems provide control of vehicle brakes by electrical signals without a mechanical or hydraulic force transmitting system between the brake pedal and the brake apply mechanisms at the wheels. Such systems may include two separate brake controllers with separate power supplies. Each control acts on two of the four wheel brake apply mechanisms, so that the loss of one of the controllers or of some of the apply mechanisms will still allow the still operating controller/apply mechanisms to stop the vehicle. Although such vehicles would normally be provided with an emergency brake such as the standard, cable operated rear brake actuator provided in all vehicles, it could also be advisable to prevent vehicle operation if less than a certain predefined portion of the brake by wire system is non-operational at the attempted initiation of vehicle motion.
Most motor vehicles with automatic transmissions are provided with a supplemental brake mechanism to prevent vehicle movement when not in use. A mechanism in the automatic transmission provides a park mode in which the transmission is locked to prevent vehicle operational movement but unlocked to permit one or more drive modes in which vehicle operational movement is allowed. In addition, most vehicles with automatic transmissions include a brake transmission shift interlock (BTSI) mechanism that prevents a shift from the park mode to a driving mode unless an activation of the brake pedal is sensed, so that the shift into a driving mode always takes place with the brakes applied. But neither of these features, as currently realized, is intended to test the brake system itself or automatically prevent vehicle operation if a brake system problem is discovered.
It is thus an object of this invention to adapt a vehicle supplemental brake system, by means of a brake transmission solenoid interlock unit, to a brake by wire system to prevent the initiation of vehicle operational movement unless a predefined portion of the brake by wire system is operational.
This object is realized in a brake by wire system for a motor vehicle having a transmission with a park mode in which vehicle operational motion is prevented and a drive mode in which vehicle operational motion is allowed and a brake solenoid transmission interlock unit effective to prevent release of the vehicle transmission from the park mode until activated. The brake by wire system has a brake intent signaling device, such as, for example, a brake pedal with at least one pedal activation indicating sensor and a brake apply mechanism. The system further has a controller programmed to activate the brake apply mechanism in response to the brake intent signaling device. The controller is further programmed to detect an operational state of the brake by wire system, for example through a diagnostic program. The controller is further programmed to generate a transmission release signal in response to simultaneous presence of the brake intent signal and detection of the operational state of the brake by wire system. The system further has a supplemental control unit responsive to the transmission release signal to activate the brake solenoid transmission interlock unit and thus permit release of the transmission from the park mode to the drive mode.
The apparatus of the invention permits different levels of control in the release of the transmission from its park mode. For example, the brake by wire system may include first and second controllers separately controlling first and second brake apply units and determining first and second operational states of the brake by wire system. In one embodiment, a transmission release signal from either one of the controllers will enable activation of the brake solenoid transmission interlock unit; but in another embodiment, transmission release signals from both of the controllers are required. In addition, the first and second operational states of the brake by wire system may be different, based on different components communicating with each of the controllers, or redundant, with a communication bus provided between the controllers to share information in determining the first and second operational states. The apparatus of the invention integrates well with the prior art brake transmission solenoid interlock systems already on most vehicles. It requires only the addition of the supplemental control unit and code in the controller(s) for determining and communicating the operational states of the brake by wire system; and it does not require the separate brake switch of the prior art brake transmission solenoid interlock systems, since the controllers already receive a brake intent signal from the sensors on the brake pedal.